The present invention relates to a method and an apparatus for detecting internal cracks, fractures, or other defects in a workpiece utilizing ultrasonic techniques with time inversion.
Ultrasonic, non-destructive testing systems are known and such are especially well-suited for testing metal workpieces to detect any internal defects. However, such known systems lack accuracy when attempting to locate a heterogeneity in complex materials, such as special alloys, composites, ceramics, or materials made by powder metallurgy. It is difficult, or even impossible, to use echo signals generated by an ultrasonic transducer in the aforementioned materials and, more particularly, in titanium, due to the presence of substantial noise echoes arising from the diffusing nature of such materials.
U.S. Pat. No. 5,092,336 discloses an apparatus for locating and focusing ultrasonic waves primarily intended for research purposes, and for the destruction of calculi in human tissue. This apparatus makes use of a phase-conjugate ultrasonic amplification procedure, also known as "time inversion". Following generation of an unfocused ultrasonic beam onto the material and receiving an echo returned to a transducer, this system inverts both the time distribution and the shape of the returned echo and retransmits this inverted signal into the material. However, this essentially static procedure cannot be used under industrial situations, particularly to detect defects in workpieces, particularly those of revolution about an axis of symmetry, since it requires complex, high-performance systems allowing short order alternation of the motions and stops required for the auto focusing feature of this procedure.